'Complain of' means 'to state that you are suffering from a certain problem, and possibly to describe it'. It is most commonly used in medicine, but isn't limited to medicine. When you go to a doctor and complain of an illness, this means that you tell the doctor that you have the illness and possibly that you describe it. The object of 'of' must be a problem of some kind: you could complain of a headache, an unreliable web server, an unfaithful spouse, mistreatment by police, etc.
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